I have flashback enabled (as sysdba) for a table on my oracle 19 database:

CREATE FLASHBACK ARCHIVE flashback_at0672 TABLESPACE users
  QUOTA 10G RETENTION 1 YEAR;
  
ALTER TABLE at0672.assays FLASHBACK ARCHIVE flashback_at0672;

When I try to alter the table the session hangs - and the table is blocked.

ALTER TABLE at0672.assays add dummy varchar2(20);

I have experienced this on different db users - and on different db server instances. No other users or sessions are logged in at that time. The lock is showing in gv$locked_object, but I could not find references, what causes the lock. I also have not been able to release the lock anyhow - except by restarting the whole database.

One common thing: All my databases are non-container databases.